1. Field of the Invention
This invention relates to a semiconductor integrated circuit device and more particularly to a semiconductor integrated circuit device having an electrically rewritable nonvolatile semiconductor memory.
2. Description of the Related Art
As a method for realizing narrow threshold voltage distribution while enhancing programming speed, a quick-pass write method is known (Jpn. Pat. Appln. KOKAI Publication No. 2003-196988). In the quick-pass write method, voltage applied to a bit line at the programming time is selectively set to one of three values of VDD, 0.5 V and zero.
That is, a voltage of zero is applied to a bit required to be strongly programmed so as to shift the threshold voltage of the memory cell to a large extent. A voltage of 0.5 V is applied to a bit required to be weakly programmed so as to shift the threshold voltage of the memory cell to a small extent. Further, the voltage VDD is applied to a bit which is not required to be programmed. Thus, the programming speed is enhanced and narrow threshold voltage distribution can be realized.
However, in the quick-pass write system, the operation of charging the bit line is performed in two stages of VDD charging and 0.5 V charging. For example, after voltage VDD is applied to a bit line which is to be charged to VDD, a voltage of 0.5 V is applied to a bit line which is to be charged to 0.5 V. Thus, the time required for charging the bit line becomes long.